1. Field of the Invention
This invention relates to an image forming method for rendering electrostatic latent images visible by electrophotography. It also relates to a process cartridge used in the image forming method.
2. Related Background Art
As a commonly available image forming method for electrophotography, a method is known in which fixed images are obtained by, e.g., forming an electrostatic latent image on an electrostatic latent image bearing member by utilizing a photoconductive material and by various means, subsequently developing the latent image into a toner image by means of a developing assembly to make the latent image visible, and then transferring the toner image onto a transfer material such as paper as occasion calls, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor.
As the developing assembly in such electrophotography, an assembly is commonly known which is so set up that a toner regulating blade made of rubber or metal, serving as a toner layer thickness control member for controlling a toner coat level, is kept in contact with the surface of a developing sleeve serving as a toner bearing member.
A toner is provided with positive or negative electric charges by the friction between such a toner regulating blade and the toner and/or the friction between the toner bearing member and the toner. Further, through the toner bearing member, which has thinly been coated on its surface with the toner by the aid of the toner regulating blade, the toner is allowed to fly and adhere to the electrostatic latent image formed on the surface of the electrostatic latent image bearing member, standing opposite to the toner bearing member, to perform development. Such a technique is commonly used in practice.
As a technical trend of image forming apparatus in recent years, it is sought to achieve further higher speed and higher reliability over a long period of time in addition to a high degree of minuteness of images, high grade images and high image quality. In order to accomplish a high-resolution and high-minuteness developing assembly system, toners having a high developing performance are being put forward to, e.g., make them have smaller particle diameter and have sharper particle size distribution.
Where a toner having such a high developing performance is used in a conventional developing assembly, the toner has tended to cause charge-up because of differences in its chargeability, powder characteristics and so forth, or to be unable to be thin-layer coated on the toner bearing member to come lacking in minuteness of images.
Meanwhile, a variety of improvements of the toner regulating blade, the toner bearing member and so forth have been attempted.
For example, Japanese Patent Application Laid-open No. 2004-4751 discloses a proposal of a developing assembly in which the surface of a developer carrying member has specific hardness and deformation degree and a developer level control blade has a surface ten-point average roughness (Rz) of from 0.3 to 20 μm on its side to be kept in contact with the developer carrying member. In this patent document, examples are disclosed in which a non-magnetic toner is evaluated by using this developing assembly, and it is demonstrated that effects on solid-image density and against unevenness, lines and so forth are brought out in every environment. Meanwhile, sufficient studies have not been made on the stability in long-term running, and, especially when a one-component magnetic toner is used, there has been a tendency of showing an insufficient running stability.
Japanese Patent Application Laid-open No. 2004-12542 also discloses a developer control member which is so set that the surface roughness of the developer control member is more than 2.0 μm as ten-point average roughness Rz and the maximum height Rmax in unevenness is smaller than the average particle diameter of the developer. As disclosed in the Examples in this patent document, in use in combination of a metallic blade and a non-magnetic toner, an effect is brought out against a phenomenon of roller set. However, where, e.g., a urethane rubber blade or a silicone rubber blade other than the metallic blade is used, there has still been room for improvements regarding running stability of developing performance and environmental stability.
Japanese Patent Application Laid-open No. 2000-330376 further discloses a proposal of a toner regulating blade in which the surface roughness of an elastic blade member is specified on its surface that is to come into contract with and rub a developing roller. This patent document discloses that the toner regulating blade of such invention is effective against thick line images and on charging stability when a one-component magnetic toner having an average particle diameter of 8 μm is used in the Examples. However, a sufficient reference is not made to developer properties, and there has still been room for improvements regarding running stability and environmental stability in an instance in which a toner having a high developing performance is used.
Besides these, Japanese Patent Applications Laid-open No. H06-186838 and No. 2004-117996, Japanese Patent No. 2986343, and Japanese Patent Applications Laid-open No. 2004-94138 and No. 2004-117919 also each disclose invention in which the surface roughness of a toner regulating blade is specified.
In all of these patent documents, the surface roughness is specified in the vertical direction, but no argument is made on the surface roughness in the horizontal direction, such as unevenness hill-to-hill intervals or hill density.
Where any of the toner regulating blades as described above are used in a high-speed developing system having an especially high process speed and making use of a process cartridge having a large volume, the problems as stated above may come to be revealed. Thus, there remains room for further studies.
Studies on improvements of toners have also hitherto attempted. For example, Japanese Patent Applications Laid-open No. H03-84558, No. H03-229268, No. H04-1766 and No. H04-102862 disclose techniques in which the particle shape of toner is made close to being spherical by production processes such as spray granulation, dissolution method, and polymerization. However, these techniques all require large-scale equipment in the production of toners, and not only are unpreferable in view of production efficiency but also have not come to sufficiently improve the developing performance of toners.
Japanese Patent Application Laid-open No. 2003-270856 and No. 2004-188725 also disclose techniques in which toners are thermally modified, which, however, have not still sufficiently well remedied image defects such as sleeve negative ghost.